Application of lasers to optical communication, optical information processing, optical processing, and the like requires optical elements having various functions, such as deflection, modulation or wavelength conversion. Optical materials having nonlinear optical effects play a central role as such optical elements.
Conventional studies on nonlinear optical materials have been centered at inorganic crystals, e.g., LiNbO.sub.3, LiIO.sub.3, KH.sub.2 PO.sub.4, and GaAs. Since these inorganic crystals, however, respond to light by their electrons participating in chemical bonding among atoms or ions and are therefore attended by lattice vibration, it is impossible to obtain a higher response rate than picoseconds. Further, the thresholds of breakage by strong laser beams are on the order of MW/cm.sup.2. In addition, since these inorganic materials are generally used as single crystals, they are insufficient in mechanical strength, particularly impact strength, and moldability for practical use.
On the other hand, organic molecular crystals, e.g., urea, p-nitroaniline (p-NA), and 2-methyl-4-nitroaniline (MNA), possess great nonlinear optical effects attributed to intramolecular non-localized .pi.-electrons. Because electron polarization of these organic materials is free from influences of lattice vibration, higher response rates and higher optical breakage thresholds are noted as compared with the inorganic crystals. Notwithstanding, they should be single crystals before great nonlinear optical effects can be attained so that it is very difficult to handle the organic molecular crystals of high performance due to dynamic and thermal weakness and, also, they find difficulty in molding into fibers or films.